This invention is directed to a focusing low perveance ion accelerator having three electrodes in the low perveance ion optical system and arranged so that voltage variation causes ion beam focusing.
The generation of high current well formed and focused ion beams is essential in ion implantation, ion microscopy and most recently in ion beam fusion where high brightness is desirable.
U.S. Pat. No. 3,955,091 defines a low perveance system wherein two electrodes, a focus electrode and an extraction electrode, each specially shaped, produce a high energy ion beam having greater intensity and less divergence and distortion than those produced by prior methods and apparatus. Such a configuration produces an ion beam current which is proportional to V.sub.o.sup.3/2, where V.sub.o is the voltage of the extraction electrode with respect to the focus electrode. Beam focus is controlled only by adjusting the plasma density in the ion source. There is thus need for additional control over beam focus and current.